NICE AND EASY: What is noise figure?

Not such an easy question actually. The term “noise figure” is tossed around a lot when you talk about antennas and amplifiers. Yet, it’s hard to understand. It’s like, everyone knows what it is, and everyone seems to talk about it like it’s so easy, but then when it comes down to explaining it, all they do is quote formulas that make no sense like:

Which doesn’t help you a whole lot when you’re standing up on the roof looking at your satellite dish.

We talked a little about noise figure in an earlier article but even that was pretty technical. Although, that’s nothing compared to this Agilent paper which takes about 7 pages to try to even define the term. All of this is great if you want a career as an RF engineer. What if you just want to decide on which amp to buy? Here’s what you want to know, in two sentences. Why is that so hard:

Noise figure tells you how much noise your system adds to a signal. You want the number to be as low as possible.

Was that so hard? I guess so.

Here’s the thing you need to know. When you amplify a signal, you also amplify the noise. Think of it this way. You see a friend across a crowded room. He starts to talk to you. So you shout “Speak up!” Everyone in the room starts talking louder.

What you really want to say is “Everyone else shut up” but you can’t do that with broadcasting. You have to take the noise when you take the signal. You can do some fancy digital things but you’ll never get rid of all the noise.

And then… even worse news… everything MAKES noise. An amplifier actually PUTS noise into a signal. How much? Is there a number that tells me that?

AHA! You’ve discovered the NOISE FIGURE. It tells you how much noise your amplifier is making. That’s why you want that number to be as low as possible.

I have a feeling there is a cadre of RF engineers prepared to explain how this article doesn’t really adequately cover the topic. Let’s talk! In the meantime, for you average folks, now you know about noise figure.

About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 8,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.